Chromium Isotope Ratio as a Tool to Track the Redox State of the Earth

The reconstruction of redox conditions in ancient oceans is a major challenge in Earth systems science. Redox conditions, commonly described in terms of the abundance of molecular oxygen relative to dissolved sulphide, are controlled by weathering reactions, volcanic emissions, biological processes, and ocean mixing. Therefore, redox reconstructions over geological timescales trace the evolution of oceanic and atmospheric chemistry and of the connections between the biosphere, hydrosphere, and geosphere. Earth’s redox state (the oxidation of the planet’s atmosphere and ocean) has played an important role in the evolution of life and, ultimately, Earth’s habitability. Therefore, understanding redox over various timescales throughout our planet’s history is an important step in determining how the Earth came to support the diversity of life we see today.

Isotopes of redox-sensitive metals have provided key information about past redox conditions at the Earth’s surface. Cr also has the potential to provide constraints on redox conditions in the oceans and atmosphere because Cr isotopes are fractionated during redox reactions,  and a number of recent studies have attempted to use this system to trace changes in the level of O₂ in the atmosphere in the Archaean and Neoproterozoic. The simplest application of Cr isotopes as a paleoredox proxy relies on the basic idea that variability of δ⁵³Cr in sedimentary rocks requires mobile Cr(VI) on Earth’s surface and that the oxidation of Cr(III) to Cr(VI), via Mn oxides, requires free oxygen.

The use of Cr isotopes as a redox proxy is limited for now because scientists haven’t built up a clear understanding of the global mass balance of Cr isotopes. Many geological reservoirs and isotope fractionation processes are still not well understood.

One environment in which this mass balance is little-understood is in estuaries. The new study now addresses this gap in knowledge by focusing on the Connecticut River estuary, collecting samples over a gradient from fresh to salty water. The study provides the first Cr isotope dataset for this type of environment. The team of scientists examined Cr-containing particulates in the water as well as dissolved Cr. Their results suggest that particles lose Cr as salinity increases, a finding that could have implications for interpreting Cr isotope data from the sedimentary rock record of the Earth. The authors conclude that interpreting δ⁵³Cr in the sedimentary record requires taking multiple processes into consideration.

The work was supported by NASA Astrobiology through the Exobiology Program.

Used techniques and instrumentation:

Thermo Scientific - Element XR™ High Resolution ICP-MS

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